1. Field of Invention
This invention relates to semiconductor index guided lasers.
2. Description of Prior Art
Single-mode index guided semiconductor lasers are known to be useful devices providing excellent beam quality and kink-free optical power vs current performance. Index guiding refers to the creation of an optical waveguide by varying the optical index of refraction between semiconductor material layers (perpendicular direction) or by varying the index of refraction via other techniques such as by the formation of a ridge or buried heterostructure forming a waveguide in the lateral direction.. Waveguides created in this way establish stable optical modes which can be incorporated in a laser structure to improve beam quality and stabilize outputs. Low divergent ridge waveguide (RWG) semiconductor lasers possessing excellent beam quality and kink-free output optical power vs input electrical current (P-I) characteristics are realized by designing the RWG to support a fundamental lateral mode. The ridges are usually formed by etching a channel into the semiconductor to depth approaching the active layer, the modal characteristics of the ridge is determined by its width and depth. This limits the width of the ridge to approximately 3-4 microns for GaAs based devices supporting a single lateral mode. Due to material limitations this limits the maximum output power, limited by the intra-cavity peak intensity. Optical densities exceeding this limit causes the material at the facets to degrade leading to catastrophic optical damage COD, destroying the laser. To increase the single-mode output power for a given intra-cavity peak intensity the fundamental mode volume is increased. Researchers have widened the optical mode in the perpendicular direction by creating broadened waveguides (perpendicular to the epitaxial layers) but are limited in width by the onset of higher order modes. Multi-mode operation is undesirable for beam quality is deteriorated and P-I performance becomes kinky.
Reliable single-mode continuous wave (cw) operation of semiconductor-diode lasers has been achieved up to powers of 100 mW, although they are readily available commercially The power limitation of these devices arises form laser facet breakdown resulting in Catastrophic Optical Damage (C.O.D). High optical powers emanating from a very small surface area push the power density at the facet to power densities exceeding the intra-cavity-power density determined by the material damage limit. Localized heating at the facets is caused by non-radiative recombination at the facet of injected carriers and absorption of photons at the facet. These effects can cause ablation of the material from the facet, facet melting and the generation of other destructive mechanisms, resulting in C.O.D.. In addition the gaussian like intensity profile of the fundamental mode causes an intensity peak in the center generating a localized hot spot which reaches the critical limit first.
Research has been conducted to try to increase the optical modal volume of index guided lasers. One approach for increasing the single mode cross section is to taper the waveguide in the vicinity of the facet (thus spreading out the optical mode). Single element semiconductor lasers tapered to 7 .mu.m at the facet were demonstrated at 600 mW. One problem with this design is that in addition to the waveguide being widened, the gain region is also widened. Thus the structure must be properly designed to filter out higher order modes.
The general belief of the semiconductor-laser community is that even with significant improvements in diode-laser lifetimes and increases in catastrophic facet-damage limits as well as improvements in thermal management, maximum single element diode laser output powers are limited to less than 1W in a single mode under cw operation. The limiting factor becomes the modal cross section, limited to approximately 4 .mu.m.sup.2, for conventional single element index guided GaAs based semiconductor lasers and power densities predicted to peak at approximately 20 MW/cm.sup.2 resulting in a theoretical limit of 0.8 Watts.
This belief has lead researchers to approach the development of high power single mode coherent operation lasers by alternative techniques. One technique to achieve high power and single mode operation is provided by laser arrays. In these designs multiple ridges are etched in close proximity to increase the laser output. Laser arrays can reach power output as high as 20 Watts. The fundamental problem with laser arrays is that their structure is much more complex and their fabrication that more complicated compared to single element semiconductor lasers. In addition their beam quality is poor compared to single-element single mode semiconductor lasers. It has been shown that in-phase evanescent-mode arrays are fundamentally unstable. Laser diode arrays support many optical modes and it is the varying overlap of the gain with the optical modal patterns which is at the heart of the problem. Array lasers have far-field patterns with a maximum of 81% power in the central lobe of their respective far-field thus limiting useful power and the coupling to optical fibers. And larger sizes of arrays increase device complexity, yields, costs, and limit the pulse response thereby limiting amplitude modulation. Master Oscillator Power Amplifiers MOPA are another type of structure utilized to achieve high power single mode laser operation. These devices push the limits of both growth and processing techniques. They also require internal or external isolators and may require additional optics and suffer from beam stability as a function of drive current.